The above-named study is a multi-centre, randomized, prospective clinical trial to determine whether EC/IC (extracranial/intracranial) microsurgical arterial anastomosis is effective in prevention of first or recurrent stroke in patients with carotid distribution atherosclerotic cerebrovascular disease. This operation is well established as a technically successful procedure with low mortality and morbidity. Approximately 1,000 patients from some 30 participating centres in the U.S., Canada, Western Europe and Japan will be entered during the first 3 years of the project. Criteria for entry are both (1) clinical (carotid distribution TIA, PRND, progressing stroke or completed stroke with mild deficit, occurring within past 3 months), and (2) radiological (appropriate MCA stenosis or occlusion, recognizable atherosclerotic lesions in surgically inaccessible portion of ICA, or ICA occlusion). Patients must be exempt from a number of exclusion categories and be able to provide informed consent. They are then randomly assigned to surgical or medical treatment groups, receive appropriate therapy, and are followed at 3-month intervals for 5 years or until an end-point has been reached. The clinical aspects of the study are coordinated from the Central Office in London, Ontario, Canada and the epidemiological aspects are directed from the Methods Centre in Hamilton, Ontario. The Project Director is Dr. H.J.M. Barnett of London.